The present invention relates to direct AC-to-AC converters in general and more particularly to converter systems of the matrix type such as disclosed in U.S. Pat. Nos. 4,642,751; 4,648,022 and 4,697,230. In this type of AC/AC converter, the AC power supply is directly converted into an AC output power supply for a load, through the use of bilateral switches inserted between the respective phases of an AC input source and grouped in relation to a common and corresponding phase line of the AC output. The bilateral switches are thus grouped per output line and controlled for conduction within each group so as to generate the output AC line voltage waveform.
The purpose of the present invention is to provide with such direct AC/AC converter a dependable overcurrent protection while using a reasonable number of current sensors despite the relatively large branching of currents among the several groups of bilateral switches involved.
In the latter regard, the prior art uses a multiplicity of current sensors to protect each semiconductor switches from a fault current, while there is a need to monitor input line currents, output faults and ground fault as well. Thus, it is desirable to use a current sensor which has a rating limited to the peak current to be monitored and is still capable of fault current detection.
Direct AC-AC converters generally require dependable overcurrent protection. This has been achieved at the present time either with an unacceptably large number of current sensors, or detection has been performed under an excessive level of overcurrent. The present invention aims at minimizing the number of current sensors while requiring only an extremely low level of current through the sensor for fault detection.